Chapter 5 Genetic Analysis of Apomixis - cimmyt

Ultras'",',," AoaIy.1s of Apomk'k O•••lop....' 59DiscussionComparisons between sexual and apomicticmegagametophytes represent an ideal systemto start dissecting the cellular and moleculardifferences that distinguish sexuality fromapomixis. Sexual reproduction is strictlydependent on the production and fusion ofhaploid male and female gametes. In contrast,apomictic reproduction is dependent on theformation of unreduced female gametophytes,the autonomous activation of the egg cell, andthe eventual fertilization of the polar nucleus(or nuclei). Regardless of the reproductivemethod, the megagametophyte forms entirelywithin the nucellar tissue and is dependent onthe sporophyte for its development andfunction. UI trastructural analysis of apomicticdevelopment has provided new, but limited,information on the developmentalparticularities of adventive embryony andgametophytic apomixis. So far, results havemainly described specific aspects ofdifferentiated embryocytes and aposporousinitials, and the fine structure of cellularizedmegagametophytic cells before and afterparthenogenetic activation.The similarity of ultrastructural characteristicsshared by adventive embryocytes andaposporous initials deserves special attention.The presence of a thick cell wall appears to beprevalent around embryocytes andaposporous initials. An extensive survey of thegametophytic-sporophytic junction in landplants reveals that the two generations of thesexual angiosperm life cycle are almostinvariably separated by thickened cell wallslacking plasmodesmata (Ligrone et aJ. 1993;Bell 1995). The presence of a conspicuousboundary surrounding the megagametophyteof some mosses and ferns of the earlyDevonian (Remy et aJ. 1993) suggests thatgametophytic cell isolation may have 3 crucialfunction of fundamental importance for theevolution of the angiosperms. Some alsosuggest that a combination of hyd ratedpolysaccharides and callose in the cell wall ofmeiotically derived megaspores may act as amolecular filter that impedes the transport oflow molecular weight peptides and/or nucleicacids expressed in nucellar cells, since theirpresence may represent a threat to the initiationof gametophytic development (Knox andHeslop-Harrison 1970). In sexual plants, thepresence of callose surrounding degeneratingmegaspores (but not viable ones) suggests thatcallose may be suppressing the nonfunctionalmegaspores by isolation. In lower plants(mosses and ferns), selective permeability ofthick wall boundaries allows only the transportof minerals and simple sugars (Bell 1988, 1992);plasmodesmata appear to be completelyabsent. Callose deposition was detected inadventive embryocytes in young Citrus seeds(Wakana and Uemoto 1987). Carman et al.(1991) compared the distribution of callose insexual and apomictic megaspores of Elymusrecticetus and found that, contrary to normalmeiosis, the cell wall of diplosporous MMCsinvariably lacks callose. These results havebeen confirmed in several diplosporous grassgenera (Leblanc et at. 1995; Peel et aJ. 1997; seealso Leblanc and Mazzucato, Chap. 9). Thedrastic cytoplasmic transformations occurringin young embryocytes and aposporous initialsmay depend on a developmental program thatis only initiated in the absence of informativemolecules originating in the nucellus. A recenthypothesis postulated by Carman (1997)suggests that the role of isolation duringmegasporogenesis may depend on expressionof duplicate genes, especially in polyploids inwhich the time and duration of specificcytological events could be the cause ofanomalies during megasporogenesis. Needlessto say, the role of cell wall thickening andcallose deposition during megasporo-genesis,diplospory, aposporous initiation, or nucellarembryony remains an unsolved problem.